The invention pertains to the fabrication of electrically conductive silicon carbide articles, and in particular to the fabrication of fully dense and mechanically strong articles.
Electrically conductive silicon carbide (SiC) articles are well known, one particularly common application being in gas igniters which use Hot Surface Ignition (HSI). Currently, gas igniters are typically made by loosely sintering relatively coarse (100 to 200 xcexcm) SiC powders to form a porous matrix, which is then infused with metallic silicon that forms an interstitial network. The electrical conductivity of the resulting composite body is due almost entirely to the interstitial silicon, and the contribution of the SiC itself is insignificant.
Such a structure is prone to fracture, so HSI articles made in this manner are likely to have a high failure rate. The electrical resistance is likely to vary considerably between articles because of inevitable variations in the way the SiC particles pack together. Also, even if the article as a whole remains intact, the interstitial silicon network may yield to internal stresses over time, which will be manifested by a change in electrical resistance. Furthermore, some of the interstitial silicon may diffuse into the SiC over time, which again will show itself as a change in electrical resistance. Additionally, the presence of voids in the final article limits its electrical conductivity to some value below the inherent conductivity of the matrix material.
Despite these disadvantages, the refractory nature of SiC is an attractive property for HSI applications. Such disadvantages would be overcome if the HSI article were made with minimal porosity and if its electrical conductivity resided in the matrix material rather than in an interstitial network. They would be further overcome if voids in the matrix material were largely eliminated, as this would lead to an improved and more consistent electrical conductivity.
Furthermore, if the HSI article owed its conductivity to a conductive dopant species distributed within the entire volume of SiC, its conductivity would be relatively stable over the lifetime of the article, since there would be no driving force to change the dopant distribution and the electrical properties of the article.
Therefore, there is a need for an inexpensive process to fabricate SiC HSI articles which will be resistant to mechanical failure and reliable in operation. There is further a need for an inexpensive process to fabricate such articles as a fully dense, single-phase SiC material which has a stable and improved electrical conductivity. Such a process necessitates minimizing the SiC particle size to accelerate the doping and sintering kinetics. The faster kinetics in turn minimize the opportunity for grain growth in the final product, thus improving its mechanical properties.
Accordingly, it is an object of this invention to provide a method for saturating the silicon carbide with dopant in order to stabilize its electrical properties.
It is another object of this invention to provide a method for making an electrically conductive silicon carbide article from dopant-saturated silicon carbide.
It is yet another object of this invention to provide that the electrically conductive article is made up of submicron-sized grains and is generally void-free.
It is yet another object of this invention to provide an inexpensive method for making such an electrically conductive article.
When a non-conductive silicon carbide (SiC) particle is reacted at a high temperature under non-oxidizing conditions with a suitable dopant source, it can be converted into an electrically conductive form by the diffusion of a dopant species from the source into the SiC. Possible dopant species include Be, Mg, Ca, Sc, Ti, B, Al, Ga, C, N, P, As, Se, S, Cl, and Br.
In the case of metallic dopant elements, the dopant source is typically a corresponding oxide.
Other refractory (i.e., ceramic) materials besides silicon carbide may be contemplated as the matrix material for practicing this invention, as will be indicated later.
Aluminum is of particular interest as the dopant species for silicon carbide. In principle, metallic aluminum could be used directly as the source. However, aluminum being highly reactive, powders with smaller particle diameters than about 0.1 xcexcm can be explosive in air and are therefore hazardous to handle.
It is also possible in principle to use aluminum oxide (alumina) as a starting material. However, although submicron alumina powders are commercially available to a degree, they are increasingly expensive and difficult or impossible to obtain as the particle size approaches 0.1 xcexcm
It has been found that aluminum powder sized at around 20 xcexcm or greater, which is easily handled, can be readily milled in an aqueous medium.
Metallic aluminum is ductile and would not normally be expected to mill satisfactorily. However, a reactive milling regime is established whereby successive layers of an alumina skin are first formed around each aluminum particle by oxidation, and then removed from the particle by abrasion to form alumina particles with diameters typically around 0.01 xcexcm.
The resulting alumina particles are mixed with submicron SiC powder along with a polymeric organic binder, and portions of the mixture shaped to form a green body. In this disclosure, the term xe2x80x9cgreenxe2x80x9d refers to a body that has been shaped but has not been further processed into a durable article. The green body is fired to temperatures high enough to reduce the alumina to aluminum, all of which diffuses into the SiC, and then to promote sintering of the doped SiC.
It is desirable to saturate the SiC with aluminum, for reasons which will become clear. In the context of this disclosure, saturation refers to a generally even distribution of the dopant species through the entire volume of a particle at some equilibrium level that corresponds to the particular processing conditions. Other factors being equal, it takes longer to saturate a large particle than a small one, since diffusion is a rate process initiated at a particle surface and the dopant species must penetrate a greater depth. Also, since the surface/volume ratio of a particle falls with increasing diameter, a given mass of particles has less available surface where diffusion can be initiated. In this context, it will also be appreciated that the alumina particles should be small compared with the SiC particles, since this will distribute the aluminum around the silicon carbide surface, thus allowing Al diffusion to start at many points around the surface. Therefore, in this invention, small (submicron) SiC particle sizes and even smaller alumina particles are selected. The kinetics of the reaction permit the aluminum to essentially saturate the SiC particle. The result is a body composed of a single-phase, Al-saturated Sic.
Along with the processing kinetics, the properties of the final article also are enhanced by using submicron particle sizes. Small raw material particle sizes lead to fine grain diameters in the final product, which are known to improve mechanical properties. Furthermore, grain growth, itself a kinetic process, occurs during sintering. The more rapid the sintering process, the less grain growth occurs. In fact, the grain sizes in the product do not significantly depart from the raw material particle sizes when the sintering time is minimized.